The present invention relates generally to a compact and low-cost illuminating device employing a light guide to provide a longitudinal, uniform and highly concentrated illumination.
Document processing devices such as scanners, fax-machines and electronic copy machines need a uniform, efficient and sufficiently intense longitudinal illumination on a target document. As a consequence of the requirement for both efficiency and intensity, a longitudinal illumination is preferred. The required illumination can be provided by a discharge tube such as a fluorescent lamp or a light-emitting-diode (LED) array consisting of a plurality of LEDs. Recently, with the advance in the LED technology and the sensor technology, the required illumination flux can be supplied by a couple of LEDs. Therefore, there is a need for an illumination device which can provide a longitudinal illumination for document processing devices by using a very limited number of LEDs.
It has been well known that a light guide such as optical fiber can guide light from a single light source to a desired location remote from the light source without encountering substantial transmission losses. Furthermore, a light guide with properly built-in light directing features along its length can be used to provide a longitudinal illumination. Illumination systems based on a light guide are formed by modifying the light guide to redirect an incremental amount of the total amount of light propagating through the guide laterally.
In general, two factors determine the distribution of illumination intensity of a device based on a light guide. The first factor is the local light flux density inside the light guide and the second factor is the local light-extracting efficiency. The amount of output light and consequently the intensity of illumination is proportional to the product of these two factors. Although a certain amount of output light is necessary for providing a certain intensity of illumination, a light-concentrating optics is further desirable to project substantially all of the all output light into a defined zone of a target plane in order to achieve a high energy efficiency and to reduce harmful scattered light.
A conventional method of increasing or reducing the local light flux density inside a light guide is to increase or reduce the local cross-section area of the light guide. However, varying the cross-section of a light guide usually eliminates or limits the possibility of integrating a light-concentrating optics into the light guide. In addition, an achievable modulation of local light flux density is limited because of possible violation of total internal reflection conditions.
In principle, the local light-extracting efficiency of a light guide can be modulated by varying the size of a light-extracting feature, for example, varying the depth or width of a reflective groove. However, width variation of a light-extracting feature as described in the prior art results in a proportional width variation of the illumination zone, which means no increase in illumination intensity despite an increase in output light flux. Depth variation of a reflective groove is accompanied by a big separation of individual grooves and consequently may result in an unacceptable high frequency intensity modulation in an illumination plane. Varying the gap between individual light-extracting features can be used to modulate output light amount as well, but this method has the same drawback as that of varying groove depth.
There are numerous methods by which a longitudinal light guide can be prepared to effect a lateral transmission of light. For example, the light guide can be cut with grooves at various points along its length, with one or more of the groove surfaces coated with a reflective material. Examples of illuminators prepared by the discussed techniques are generally disclosed in U.S. Pat. Nos. 4,052,120 issued to Sick et al.; U.S. Pat. No. 4,172,631 issued to Yevick; U.S. Pat. No. 4,173,390 issued to Kach; and U.S. Pat. No. 4,196,962 issued to Sick. Alternatively, grooves with profiles other than triangles and without using a reflective material can be used in a light guide as disclosed in U.S. Pat. No. 5,835,661 issued to Tai et al.
While illuminators prepared using techniques disclosed in the above-mentioned patents may provide some lateral light emission along a light guide, the illumination is generally divergent and a further control of illumination uniformity as required by document reading devices is not possible. Some prior art designs have tried to provide a means to concentrate illumination. See, for example, U.S. Pat. No. 2,825,260 to O""Brien which shows a triangular light guide, amongst other shapes; U.S. Pat. No. 4,678,279 to Mori which shows a modified cylindrical light conducting member; and U.S. Pat. No. 5,295,047 to Windross which uses an integral optical lens together with a light guide pipe having an isosceles triangular cross-section. Nevertheless, the light guides shown in these prior patents are generally not capable of being used to illuminate a longitudinal area with a sufficiently uniform intensity.
To achieve a good illumination uniformity, U.S. Pat. No. 5,808,295 issued to Takeda et al. and 5,905,583 issued to Kawai et al. use a light guide with variable cross-section and place a light source deviated sideward from the normal line passing through a center of the reflection area of the light guide. While the designs according to these prior patents improve the illumination uniformity, using variable cross-section also limit the possibility of using a light concentration feature to control the width and position of an illumination zone or achieve a highly concentrated illumination. Furthermore, placing a light source deviated sideward from the normal line of the reflection area constrains the freedom of LED packaging and assembly of LED to a light guide.
In addition, none of the designs according to the prior patents provides a simple and adequate assembling means for an illumination device to be integrated into a document processing device.
There thus exists a long felt and unresolved need to provide an illumination device that overcomes the above-described short comings of the prior art.
The present invention is directed to an illumination device that advantageously provides, in a novel and unobvious way, a substantially longitudinal, uniform, and concentrated light output. The illumination device is preferably constructed as a unitary structure including a light guide having first and second optically coupled sub-guides. A light-extracting feature is optically coupled to the first sub-guide and spaced apart from an entrance opening of the first sub-guide. The light-extracting feature redirects light within the light guide to form an effective light-emitting object at the entrance opening. Light from that light emitting object is projected out of the light guide by light-concentrating optics provided by the internal surfaces of the first sub-guide. The first sub-guide has a predetermined cross-sectional shape and a substantially uniform cross-sectional area along the longitudinal length of the light guide. The second sub-guide also has a predetermined cross-sectional shape but has a varying cross-sectional area along the longitudinal length of the light guide that controls light flux density within the light guide. An illuminating device constructed in accordance with the present invention thus provides a highly uniform illumination output with a high grade of light concentration, facilitates easy assembly, allows more freedom in light source packaging, and may be manufactured at a relatively low cost.
A light guide constructed in accordance with the present invention separates the functions of concentrating illumination output and controlling local light-flux intensity into first and second sub-guides optically coupled to each other. The light-extracting feature of the present invention generates an effective light-emitting object having a constant width for a light-concentrating optics. Illumination intensity output of the inventive illumination device may thus be modulated by varying the width of a light-extracting feature without affecting the width of the output illumination zone.
The first sub-guide of the light guide of the present invention has a predetermined cross-sectional shape and a generally constant area along its longitudinal length (i.e., along the longitudinal length of the light guide). The first sub-guide has an entrance opening through which light extracted by a light-extracting feature may pass and an output surface for emitting light output from the device. At least part of the internal peripheral surfaces of the first sub-guide also functions as light-concentrating optics to project light extracted by the light-extracting feature to an illumination target through the output surface.
In one embodiment of the present invention, the first sub-guide has a generally bell-shaped cross-section with a wider and a narrower opening. The wider opening generally comprises the output surface of the illumination device and the narrower opening generally comprises an entrance opening between the first sub-guide and a light-extracting section. The bell-shaped cross-sectional profile comprises two curved side surfaces and a curved output surface closing the wider opening of the bell-shape. Those surfaces work cooperatively as light-concentrating optics for projecting output light onto an illumination target. The narrower opening of the bell-shape is optically connected to a light-extracting section, which provides a light-extracting feature to redirect light striking thereon towards the narrower opening to form an effective light-emitting object for the light-concentrating optics. Cut-off of the correlation between the mechanical width of a light-extracting structure and the width of the illumination zone is achieved by locating the light-extracting feature a sufficient distance from the position of the effective light-emitting object.
The illumination uniformity provided by the inventive illumination device may be further controlled by providing a second sub-guide as part of the light guide and having a variable cross cross-section. The cross-section of the second sub-guide varies gradually along the longitudinal length of the light guide so that the local light flux density can be adjusted for a desired uniform illumination. When designed in accordance with the present invention, a second sub-guide can have a cross-section shape convenient for other purposes, such as, for example to facilitate the assembly of the light guide.
When LEDs are used as light source, the point-like characteristic of LEDs can cause a non-uniform illumination at a location close to the light-input end of the light guide. To offset this, the light guide of the present invention may employ a light-homogenizing section which improves the illumination uniformity of the light source by modulating characteristics of input light or shifting the propagation path of input light sideward without physically relocating the light source sideward from the central normal line of the light extraction feature.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure hearing, and the scope of the invention will be indicated in the claims.